This invention relates to a green emitting projection cathode-ray tube of high luminance having an excellent color tone. A color having an excellent color tone represents a color having both excellent hue and excellent saturation. In the CIE chromaticity diagram, the green range lies in the range of x&lt;0.350 and y&gt;0.550. The term "excellent color tone" means that x is as small as possible below 0.350 while y is as great as possible above 0.550. FIG. 1 of the accompanying drawings is the CIE chromaticity diagram representing the chromaticity of various green emitting phosphors inclusive of the phosphors of the later-appearing embodiments of the present invention. The directions of excellent hue, excellent saturation and excellent color tone are represented by reference numerals 11, 12 and 13, respectively, in the diagram.
An InBO.sub.3 :Tb (indium borate activated by terbium) phosphor is a green emitting phosphor which can withstand electron beam radiation of a high density large current value, is suitable for a projection cathode ray tube and has an excellent color tone, but has a long decay time. Therefore, in order to use this phosphor for the projection cathode-ray tube in practice, it must be mixed with another phosphor having a short decay time and highly resistant to electron beam burning so as to shorten substantially the decay time.
It is known from the prior art references such as Japanese Patent Laid-Open No. 76585/1986 to use this phosphor by mixing it in such a manner that the decay time value is below 10 ms as measured by raster scanning method. However, when such a green emitting projection cathode-ray tube is mounted actually to a projection television set and evaluated, very gradual decline of delay is so marked that the cathode-ray tube cannot be put into practical application.
The green emitting phosphor described in the prior art reference described above, i.e. Japanese Patent Laid-Open No. 76585/1986, is prepared by adding large amounts of InBO.sub.3 :Tb to Y.sub.3 Al.sub.5 O.sub.12 :Tb in order to improve the color tone of Y.sub.3 Al.sub.5 O.sub.12 :Tb.
It is known, on the other hand, that luminance saturation is not sufficient to use Y.sub.3 Al.sub.5 O.sub.12 :Tb phosphor for the projection cathode ray tube, and Japanese Patent Laid-Open 101175/1985 instructs, for improving it, that large amounts of Al of Y.sub.3 Al.sub.5 O.sub.12 :Tb should be substituted by Ga (the molar ratio of Al/Ga being from 0.9/4.1 to 3.3/1.7). Furthermore, Japanese Patent Laid-Open No 125479/ 1981 discloses a Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Tb system green phosphors (the molar ratio of Al/Ga being from 2/3 to 3/2). However, none of these Y.sub.3 (Al,Ga).sub.5 O.sub.12: Tb system green emitting phosphors has entirely satisfactory color tone.
Incidentally, the raster scanning method described above allows the measurement of the decay time in the state that an electron beam of regular intensity scans the phosphor screen and the value of the decay time is stipulated as the decay time to 10% of peak brightness. In this case, an anode voltage is 28 kV, a cathode current is 350 .mu.A, an electron beam diameter is 0.25 mm and a beam scanning rate is 0.2 cm/sec.
As described above, the problem with the InBO.sub.3 :Tb phosphor is its high value of decay time. The result of the actual evaluation test by the projection tube using the InBO.sub.3 :Tb phosphor demonstrates that its decay time is longer than that of a green emitting projection tube whose phosphor screen is formed by use of the conventional green emitting phosphors at a 9:1 mixture rate of Y.sub.3 Al.sub.5 O.sub.12 :Tb and Zn.sub.2 SiO.sub.4 :Mn.